Allylation of carbonyl compounds mediated by zinc

Unlike the metallic tin-mediated allylation of carbonyl compounds, which requires either acidic medium, aluminum additive, ultrasonic radiation or heating, the metallic zinc-mediated allylation proceeds at room temperature. 

Substituted allyl bromides always reacted at the more-substituted carbon atom. Poor diastereoselectivities, nonetheless, were obtained with crotyl bromide or 3-chloro-l-butene. The scope and limitations of the reaction were carefully investigated. Cinnamyl bromide yielded phenylpropene and the aldehydic co-reactants remained unchanged in the absence of the halide, some ketones dimerized to the pinacol in a slow, low-yield reaction (Einhom and Luche, 1987). 

With functionalized allyl bromides, such as ethyl (2-bromomethyl)acry-late, the zinc-mediated allylation in a saturated aqueous NH4CI/THF mixture under reflux provided a-methylene-y-butyrolactones in good yield (Mattes and Benezra, 1985)  

Wilson and Guazzaroni (1989) devised a modification of the Luche reaction by using a solid organic phase, such as reverse-phase C-18 silica gel, instead of tetrahydrofuran as the cosolvent. The solid organic phase can be reused and disposal of the solvent after the reaction is environmentally safe. Unfortunately, the synlanti-selectmty was not improved regarding the Luche conditions. No chelation control was observed with p-alkoxyaldehydes. 

More recently, a promising new technique based on the combined use of pulsed sonochemistry and electrochemistry was applied for the production of submicron crystalline metal powders. The first results showed that the zinc powder elaborated by such a method was about three times more reactive than the commercial one in an aqueous Barbier-type reaction excluding any organic cosolvent (Durant et al, 1995). 

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